Circle obviously, to both colour and prettiest. Circles just flow nicer and don't have any ungainly corners. Much more organic and streamlined, circles are beautiful. Though it's difficult to develop and push the boundaries of circles. Rectangles may not look as good, but there's more scope for development for sure, and you can certainly push the limits within the constraints that define a rectangle.

_________________"You are the universe expressing itself as a Human for a little while..."